1. Field of the Invention
This invention relates to an idling engine speed control unit for controlling engine speed at the time of idling.
2. Description of the Prior Art
The engine speed is controlled to a predetermined idling engine speed in an idle running state. This idling engine speed is set to a low speed to minimize fuel consumption at the time of idling and swift and high-accuracy controllability is required to suppress variations in engine speed caused by disturbance. A conventional idling engine speed control unit is disclosed by Japanese Patent No. Sho 61-53544, for example. This unit detects an engine speed at the time of the idling of an engine, adjusts the amount of an air-fuel mixture to be supplied to the engine so that the engine speed agrees with a target engine speed predetermined according to engine operation conditions, obtains a difference between the actual engine speed and the target engine speed and suppresses a change in engine speed by correcting the ignition time to eliminate this difference.
The factors of causing changes in engine speed at the time of idling include load changes caused by turning on and off a load, changes in air/fuel ratio and the like. Conventionally, the engine speed has been controlled to a predetermined idling engine speed by detecting a change in engine speed and feed-back controlling the ignition time and the volume of intake air of the engine.
However, the feed-back control of the engine speed as in the prior art has such a problem that the beginning of a change in engine speed or a small change cannot be handled because a feed-back operation is started when a change in engine speed exceeds a predetermined value. When the control of the air/fuel ratio is carried out at the time of idling, a change in engine speed cannot be absorbed by the above-described control method because a change in engine torque caused by a rich or lean air/fuel ratio is large.